Field of the Invention
The present invention relates to an image forming apparatus which performs image formation while continuously conveying sheets, such as a copying machine and a printer.
Description of the Related Art
Conventionally, in an image forming apparatus such as a copying machine and a laser beam printer, the recording position of an image with respect to a recording sheet sometimes deviates due to variations in sheet conveying accuracy, positional accuracy of a toner image to be recorded and so on. Therefore, various techniques have been proposed regarding the registration of the toner image with respect to the sheet.
For example, the technique is proposed by Japanese Patent Laid-Open No. S60-120369 in which the registration is carried out in the configuration in which a sheet detection sensor is disposed in the sheet conveying path, and once a sheet reaches the detection sensor, the pair of registration rollers is temporarily stopped, and the pair of registration rollers is driven for conveying again taking into consideration the arrival timing of the formed toner image.
In addition, the technique has been proposed by Japanese Patent Application Laid-open No. H03-36559 in which an image is formed with the timing based on a detection signal from a sheet detection sensor, and the formed toner image is transferred to a sheet.
In these techniques, it is common to design the apparatus such that the feeding control of sheets from a sheet stacking portion at the timing as early as possible and a large margin of delay for sheet conveying from the sheet stacking section to the pair of registration rollers is allowed. This is because the sheet jam is suppressed by presuming a reduction in conveying efficiency caused by roller slip due to abrasion or paper dust adhesion of rollers up to the registration roller pair and separation failure due to the sheets on the sheet stacking portion sticking to each other.
However, when the control for starting the sheet feeding at the timing as early as possible is performed, if the image formation interval becomes large by for example adjustment control for stabilizing the color tone, the sheet conveyed to the pair of registration roller has to wait there for a time longer than usual. In such a case, special paper such as coated paper has a possibility that the surface of the sheet is deformed due to long-time roller gripping and image defects occurs due to transfer failure of the deformed portion. Similarly, when a standby state continues for a long time with a sheet being in contact with the registration roller, there is a possibility that a loop shape is made and remains at a tip portion of the sheet thereby causing a transfer failure.